Re: Sodium Bisulfite

From: Katharine Thayer ^lt;[email protected]>
Date: 08/26/04-03:34:11 AM Z
Message-id: <>

Okay, let's see if I've got this straight.

(The task I'm avoiding at the moment is carrying about 20 linear feet of
books out to the studio. I keep my art books in the studio, but I read
them in the house, so they keep gravitating into the house until there
are piles all over, and then I have to carry them all back out there
again. So I decided to see what's happening on the list instead.)

So here's what I understand from the chemists:

All the sulfites are basically the same functionally, and should work
the same at roughly the same concentration, and should smell the same,
unless the pH is different.

My experience with sodium metabisulfite is based on one batch only; I
found it more difficult to dissolve than sodium bisulfite because of the
"plates" or wafers that it formed, and it was ineffective at 5%; I had
to mix it at many times 5% to get any clearing effect. (And "freely
soluble" seems right; I put the entire 100 g into maybe 500 ml water.)
And while its smell was similar in quality to that of sodium bisulfite,
it differed in intensity; the fumes didn't knock me over and make me
cough like the fumes of sodium bisulfite. So what is the meaning of all
that, if the two things should be equivalent? The dry material came in a
sealed plastic bag and I kept it sealed until I mixed it, but I'm
beginning to think, on the basis of what I'm hearing, that there was
something wrong with my batch.

Specifically, I'm thinking that perhaps some water got into the material
(the way the stuff was in wafers instead of powder sort of suggests
that) and rendered it less effective by oxidizing it to sulfate. (I
haven't had that happen with sodium bisulfite). So I'm retracting any
statement I've made here about the difference I've observed between the
sodium bisulfite and metabisulfite; I think that observed difference was
probably spurious and not useful to the discussion.

It sounds like chemists seem to agree that there's not much difference
functionally between the different sulfites as far as clearing. So far I
haven't heard any chemists specifically address the assertion that
metabisulfite solution will wash out of the paper faster than bisulfite
solution. I still don't see why it would, and will be skeptical until
shown some data. In the meantime, I will continue to wash my occasional
cleared prints for 20-30 minutes, and would do that even if I were using
metabisulfite, as no one has shown me yet that the *belief* that
metabisulfite solution washes out faster is based in fact or even logic.

I've just got another note from my chemist, who says "You are quite
right that the solubility of the compounds in water cannot be a factor"
[in washout times for the solutions]. There's more, about ion exchange
properties and stuff, but I'll need to read it more carefully before I
understand what all it says, and right now I've really got to move some


Martin Angerman wrote:
> Here's the math:
> Formula weights are potassium metabisulfite=222, sodium metabisulfite=190,
> sodium bisulfite=104. Remember that you get two bisulfites from every
> metabisulfite. There was no listing for potassium bisulfite.
> A 5% solution of sodium bisulfite=0.48M The equivalent molar concentrations
> are 5.33% potassium metabisulfite and 4.56% bisulfite. All are close enough
> for 5% to work in clearing baths for most situations.
> The potassium salts are listed as pharmaceutical antioxidants, and the
> potassium salt as the food additive, and wine preservative (there may be a
> cost-effective opportunity for bulk purchases here). The metabisulfites are
> listed as "freely soluble" in water. The sodium bisulfite is listed as
> soluble in 3.5 parts water (~30% solution). pH may affect the ability to
> dissolve.
> All are listed as giving off a sulfur dioxide odor. They pick up water from
> the air, and oxidize to sulfate (and are then ineffective). Keep them dry
> and tightly closed. As far as the smell of a solution goes, check the pH.
> A solution is acidic. The acid form is volatile, and therefore smelly. Get
> some test strips or an inexpensive meter. I don't know the ideal pH. With
> bulk and technical grades, this may be the source for differences from type
> and batch differences. Remember, the difference between a bisulfite
> solution, and a sulfite solution (as in developer) is just the pH.
> ----- Original Message -----
> From: "Katharine Thayer" <>
> To: <>
> Sent: Wednesday, August 25, 2004 1:51 PM
> Subject: Re: Sodium Bisulfite
> > Katharine Thayer wrote:
> > >
> >
> > >
> > > But the problem is that it's sodium bisulfite and *potassium*
> > > metabisulfite that are at issue in this discussion, so how similar are
> > > they? My chemist consultant says that he doesn't see a role for the
> > > potassium and sodium, so as far as he's concerned they are spectator
> > > ions in the reduction. If this is correct, then it's only the sulfite
> > > that matters, and metabisulfite should perform about the same regardless
> > > of whether it came into solution along with potassium ions or sodium
> > > ions. If that's so, then why would we expect big differences in how they
> > > perform in the clearing and washing process?
> > >
> >
> > I just re-read my chemist's message, and found that the conclusion I
> > came to by reasoning my way through a logical sequence, he actually
> > said straight out to start with: "I wouldn't think there would be any
> > difference between sodium and potassium
> > metabisulfites at the same molar concentration." So maybe I should just
> > read my mail more carefully and give my brain a rest. Any chemists
> > disagree with this, please say so, and support your answer. Thanks,
> >
> > kt
> >
Received on Thu Aug 26 10:30:10 2004

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